Yes, you can generate image thumbnails for remote PDF documents when using Cloudinary's 'fetch' URLs by adding the 'format' ('f' for URLs) parameter. For example, adding 'f_jpg': http://res.cloudinary.com/demo/image/fetch/w_150,h_150,c_thumb,q_95,f_jpg/http://bit.ly/V896LZ
Note that requesting the generation of the thumbnail also uploads the remote PDF to your account while setting its type to 'fetch'. The thumbnail itself is created as a derived image.
You can read more about PDF handling (for e.g.: converting multiple images into a PDF and extracting images from PDF pages) with Cloudinary here.